The ErbB signaling network has multiple roles in development and homeostasis, and its components are frequent and important targets of deregulation leading to cancer. The network comprises multiple polypeptide ligands from the EGF and neuregulin subfamilies and four related receptor tyrosine kinases, ErbB 1 -ErbB4, that homo- and heterodimerize. A subset of EGF-related ligands activates ErbB4 as well as EGF-R. To investigate roles for the ErbB signaling network and interactions among its components we have systematically inactivated genes encoding EGF family ligands. During the previous funding period, studies of the derived mice revealed unique and important roles for EGF-like ligands in the developing and pregnant mammary gland, and in the gastrointestinal tract. Recently, we demonstrated that HB-EGF-/- pups frequently die shortly after birth, displaying defective cardiac heart valves and lung development, and hearts of survivors are often enlarged. We also discovered that additional loss of HB-EGF from viable mice lacking three other EGF family ligands produces embryonic lethality. Finally, similarities in the phenotypes of mice separately lacking TGFa, HB-EGF and the recently discovered disintegrin-metalloproteinase, TACE, led us to propose that TACE is a physiological convertase for both TGFa and HB-EGF, a proposal subsequently supported by biochemical evidence. We now propose to extend these studies, addressing fundamental issues regarding EGF family ligands and the ErbB network. Our specific aims are to: (1) identify phenotypes underlying the embryonic lethality of quadruple null mice, and determine if defects are consistent with known actions of EGF-R or ErbB4; (2) determine mechanisms responsible for the HB-EGF-null heart and lung phenotypes; (3) test the hypothesis that HB-EGF has unique actions compared to other EGF family ligands, as well as the possibility that its physiological actions are mediated in part through additional ErbB receptors; and (4) evaluate the requirement for soluble versus membrane-anchored forms of HB-EGF.